1. Field of the Invention
This invention relates generally to semiconductor processing, and more particularly to methods and apparatus for lithographic patterning of structures.
2. Description of the Related Art
The fabrication of modern integrated circuits requires the patterning of millions of different types of regions on a semiconductor wafer, such as local interconnect trenches, global metallization layers, and transistor gates, to name just a few. The manufacture of such multitudes of tiny structures is made possible by the use of lithographic processing. In photolithographic processing, a layer of photoresist material is applied to the wafer, frequently by spin-coating. Next, the photoresist layer is exposed to an actinic radiation source, such as deep ultraviolet (“DUV”). The DUV radiation is first passed through a mask or reticle that selectively transmits some of the DUV radiation while blocking other portions so that only preselected portions of the photoresist are exposed to the radiation. The radiation transmitted through the reticle then passes through one or more reduction lenses before striking the resist layer. The radiation changes the chemical character of the photoresist, either rendering it soluble or insoluble in a subsequent solvent step, depending upon whether the resist is negative-tone or positive-tone photoresist. The resist is then developed by exposure to a developer solvent. The areas of the photoresist remaining after the development step mask and protect the substrate regions that they cover.
The quality as well as the minimum feature size of the developed image depends on, among other things, the wavelength of the exposure radiation, the focusing capabilities of the reduction lens(es), and the optical properties of the resist and the films underlying the resist. The wavelength of the exposure radiation has historically been somewhat constrained by the requirement that illumination sources produce radiation at a relatively high intensity over a relatively narrow bandwidth. More capable reduction lenses have helped to push imaging to the diffraction limit. However, there are still physical limits to the improvement of reduction lenses.
Various authors have described the use of a solid immersion lens to image microcircuit structures. In one variant of the concept, a single solid immersion lense with a relatively high refractive index is positioned between an objective lens and the structure to be imaged. With the solid immersion lens in place, light transmitted from the objective lens is focused in the solid immersion lens instead of air or some other medium. This produces an attendant increase in the effective numerical aperture of the imaging system due to lowered effective wavelength, since the light is focused in the solid immersion lens where the refractive index is relatively high. Another conventional imaging system uses an array of solid immersion lenses to image Raman scattering from a device undergoing inspection. Whether in single or array format, known conventional solid immersion lenses have been described in the context of imaging but not lithography.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.